This invention relates to an integrated circuit for a preamplifier having a high S/N ratio and interchangeability.
Generally, the S/N of an amplifier is determined by the S/N of the first stage when the first-stage gain is sufficient. However, in order to improve the first state S/N, the resistance to being the noise signal source must be reduced. Consequently, when the impedance of the circuit is small, the noise caused by the first stage active elements causes a problem. Normally, the large noise signal source of an active element is the base spreading resistance rbb' in a transistor and the mutual conductance gm in an FET. In order to make a low-noise amplifier such as an equalizer amplifier, high hfe (current increasing rate), low rbb' transistors or high gm FETs are required as the first stage active elements.
However, it is extremely difficult to manufacture active elements such as transistors of FETs that satisfy these requirements. Furthermore, to integrate these in an integrated circuit is even more difficult. Also, when active elements, for example, transistors, are integrated in an integrated circuit, they cannot be replaced when improved transistors become available later or be replaced with FETs if desired. Thus, they also have the shortcoming of poor interchangeability.